by Solar Flare » 06 Jun 2019, 02:36
You lose a unit, but it doesn't necessarily need to be the unit that you built by gaining the SC you now lost... and if you gain an SC elsewhere you do not lose a unit.
After spring and fall turns there is a count of units vs SCs. If you have more SCs than units, you choose to build more units in home SCs (unless you're playing Build Anywhere/Age of Empires/Chaos, when it doesn't matter where you build). If you have less SCs than units you choose to destroy units to bring those numbers in line with each other. You can choose whichever unit(s) you want to destroy.